An early Devonian age for the Bulls Head Formation, Lower Old Red Sandstone, Dingle Peninsula

Abstract

The Bulls Head Formation is the oldest lithostratigraphic formation in the 2.8km thick Dingle Group (Lower Old Red Sandstone) succession of the Dingle Peninsula. The formation mainly comprises thinly bedded, fine-grained purple-grey sandstones interlaminated with thin mudstone layers (Heterolithic Member) that were deposited in a very extensive shallow lake, here named ‘Lake Blasket’. The formation also includes two lake margin sheetflood systems (the Boat Cove and Trabane Members) that are only developed on the northern lake limit. The Bulls Head Formation has been previously assigned a late Silurian (late Ludlow-Pridoli) age. Two spore assemblages obtained from the Trabane Member are assigned to Z Interval Zone of the Breconisporites breconensis and Emphanisporites zavallatus BZ Oppel Zone, which indicates the upper part of the Bulls Head Formation is early Devonian (late Lockhovian) in age. In addition, a small disarticulated fish plate is recorded from the matrix of a mudclast conglomerate of the Trabane Member and this represents the first body fossil ever to be found in the Dingle Group. The plate is assignable to the Heterostraci group of jawless fish that were common in late Silurian and early Devonian times. A new palaeoenvironmental model for the Trabane Member proposes fluctuating lake levels and periodic fluvial sheetfloods with contemporaneous erosion.